I am surprised that author was surprised about the "teacake" being sweet. I am not native speaker and yet it seems obvious to me. Cake is something sweet by design for few centuries already. Even if that's not the case, teacake's name suggest it's consumed with tea. If that assumption is correct, it's pretty common, in many cultures around the world, to have something sweet with tea to contrast the bitter flavor. Does it work differently in UK?

When you think about it, author doesn't discover anything new. It is common in any country big enough to get few regions and relatively rich cuisine to have a different/regional names for the same or similar dishes. Staple food (like bread) is the best example because, usually, it is ubiquitous in certain area.